Editor's note: Julian E. Zelizer is a professor of history and public affairs at Princeton University's Woodrow Wilson School. His new book, "Arsenal of Democracy: The Politics of National Security -- From World War II to the War on Terrorism," will be published this fall by Basic Books. Zelizer writes widely about current events.

PRINCETON, New Jersey (CNN) -- Democrats are getting nervous about the way that events have been unfolding this summer.

Although few Democratic officials have concluded that it's time to start panicking, there are disturbing political trends developing right at the time that members are starting to think seriously about the midterm elections.

The outlines of a significant political problem have emerged: the possibility that the White House simultaneously disillusions liberal Democrats and angers Republicans.

The situation is of course very much in flux. Much of what happens will depend on the decisions of the White House and Congress in September and October.

For Democrats who were working in Washington during the early-1990s, this would be déjà vu all over again. This was the problem that President Clinton suffered through in the summer and fall of 1994. Though we often remember Clinton as a pure centrist, his disagreements with the left were not inevitable.

Clinton had attracted significant liberal support in his presidential campaign. During the 1992 primaries, he championed issues that were crucial to liberals such as unemployment relief, health care reform, and gays in the military.

Clinton's election, combined with Democratic control of Congress, was perceived by most Democrats to be a promising opportunity. "The Democrats came out of the political wilderness on Tuesday," New York Times reporter Robin Toner wrote the day after the election. "The Republicans entered into it."

But Clinton's relationship with liberals soured in his first year as a result of a series of compromises that angered the liberal wing of his party. Working with Republicans in Congress, he pushed for the passage of the North Atlantic Free Trade Agreement, despite the fears of Democrats from industrial states that NAFTA would result in job losses.

Clinton settled on a tortured compromise -- "Don't ask, don't tell" -- with regard to gays in the military. And finally, Clinton's health care proposal in 1993 had abandoned the single-payer model that had been a central goal of liberals such as Sen. Edward Kennedy.

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By the fall of 1994, after health care reform was dead, liberal Democrats did not have much love left for Clinton. The president complained that he was not receiving "one damn bit of credit" from the "knee-jerk liberal press."

At the same time, Republicans were feeling revitalized and motivated by the fall of 1994 following the debacle of their losses in the 1992 election. There were many factors behind the resurgence of the GOP, ranging from the corruption scandals facing congressional Democrats like Dan Rostenkowski, to an exciting team of Republican leaders such as Newt Gingrich. But one of the most important factors was the party's ability to rally against Clinton. He and his health care proposal provided the party with the enemy they needed.

Through a sophisticated media strategy, Republicans were able to frame the health care battle as a debate about big versus small government rather than one about how to best fix a broken health care system.

As Republicans went into the midterm elections, they used the health care proposal as a symbol to paint Clinton as a classic tax-and-spend liberal -- at the exact same time, ironically, that liberals had become frustrated with the White House. Republican turnout was up; Democratic turnout was down. The results were devastating to Democrats. Republicans won control of Congress for the first time since 1952. The conservative revolution had finally come to Capitol Hill.

President Obama clearly does not have to suffer the same fate. Democrats are in better condition today than they were in 1994. President Obama's standing in the polls remains much stronger than Clinton's was at that time and the contentious battle over health care is taking place much earlier in his term. If the economy continues to improve, so too will the position of Democrats. Republicans, moreover, are in much worse shape. Democrats are likely to retain control of Congress in 2010.

Nonetheless, Democrats have reason for concern. Most importantly, a demoralized GOP has rallied through their opposition to the health care proposal as well as the deficits that will result from Obama's other domestic initiatives. There has even been some evidence that their arguments are catching the attention of independents.

At the same time, liberals have become increasingly frustrated that the administration has continued to respond to the demands of centrist Democrats, first by cutting the economic stimulus package in February and now possibly with major compromises on health care. They have watched the public insurance option appear to fall off the table and they are now hearing proposals to reduce the size of subsidies for low- and middle-income Americans to help pay their health insurance premiums.

Many of Obama's strongest supporters have been highly critical of the administration for not breaking with most of President Bush's counterterrorism policies. They are keeping a nervous eye on the mission creep that has been taking place in Afghanistan.

At a minimum, the president needs to insist on a good deal in the eyes of a sizable majority of the Democratic Party on health care if he is he is to break these trends. The most tempting legislative option will be to agree to the proposal of the Senate Finance Committee, but that could bring Democrats the greatest political risk.

The extremely watered-down compromise from that committee would be deeply upsetting to a broad range of liberal health care reform advocates, who have been expecting a much bolder transformation of the health care system.

Yet the legislation would do little to calm the Republican opposition that has emerged. While a separate, more liberal bill from the Senate Health, Education, Labor and Pensions Committee has no chance of passage, Obama must now define a satisfactory point in between.

It is easy to envision how by next year Obama can end up without anyone very excited about his agenda, other than his opponents.

The time has come for the president to inject himself into the fragmented legislative process and to draw some lines in the sand. He needs to say what is non-negotiable and at which point he would rather not have legislation than sign a bad bill. If he doesn't, Democrats could see the size of their majority diminish beyond comfortable margins and find themselves looking at a legislative environment that is even more difficult than it is now.

The opinions expressed in this commentary are solely those of Julian Zelizer.